The present invention is directed to a multi-fiber composite for making an endodontic reamer (dental file) for use in reaming, cleaning, and other preparation techniques in dental root canal procedures. More particularly, the multi-filament composite provides for a predominantly twisted or other off-axis oriented fiber construction (as a composite) at least partially embedded in a polymer matrix that can effectively transfer twisting (torque) loads, improve flexibility, and be highly resistant to complete (gross) breakage within the tooth canal.
The tooth contains one or more circulatory and neural canal systems, terminating at each root. This canal is narrow, tapered and curved to varying degrees. The pulp tissue within the canal can become diseased. To avoid the need for extraction, the diseased tissue can be removed using endodontic files and reamers and the cleaned canal sealed. These endodontic cleaning instruments are tapered and have surface features (helical recesses, etc.) designed to remove the diseased pulp and other tissue within the canal via reciprocating or rotating motion. There are numerous patents that describe various flute, helix and other design features of root canal files and reamers including U.S. Pat. Nos. 5,735,689, 5,980,250, 4,299,571, 5,882,198, and 5,104,316 by McSpadden; U.S. Pat. Nos. 4,611,508 and 4,536,159 by Roane; and U.S. Pat. No. 4,538,989 by Apairo and Heath. Other design concepts include the use of tubular devices such as those described by Kronman and Goldman in U.S. Pat. No. 4,135,302 and by Gonser in U.S. Pat. No. 4,505,676.
Because of the narrow, curved nature of the root canal, repeated bending of the file induces high stress on current files made from stainless steel and other metals. As the modulus of elasticity (stiffness) of the metal increases, the stress on the file increases for a given curvature. The modulus of stainless steel, for example, is 30 msi. Although the inherent material strength can be quite high and often is above 200 ksi tensile, the recesses (helical) along the file length act as stress risers and can create a crack that propagates across the diameter of the file, leading to breakage. Breakage of the tip of a file creates a problem in removal of the imbedded broken piece. Typically a trephine-type device, as described Ruddle in U.S. Pat. No. 5,879,160, is needed to remove the broken pieces. This adds time and cost to the procedure and additional discomfort to the patient.
When the file or instrument is rotated within the canal, a repeated cyclic stress is imposed on the instrument. The number of rotations (or repeated stress events) of the instrument that occur until breakage is referred to as the fatigue life of the instrument. The issue of improved flexibility and longer fatigue life (rotations to breakage) of dental files has resulted in a more recent group of patents for lower-modulus titanium alloy files. These include U.S. Pat. No. 5,125,838 by Seigneurin and U.S. Pat. No. 5,984,679 by Farzin-Nia and Otsen. Titanium alloys have a much lower modulus (stiffness) than stainless steel and most have an elastic modulus of about 16 msi. Alloys such as super-elastic NiTi can accommodate exceptional displacements with a relatively lower imposed stress, and thus exhibit longer fatigue lives (Nickel-Titanium Instruments, Serene, et. al., 1995). The effective elastic modulus of NiTi alloys is in the range of 4-8 msi, half that of most standard titanium alloys. But NiTi alloys can cyclicly soften during repeated load cycles, which quickly change its initial mechanical characteristics (Ritchie and McKelvey, xe2x80x9cFatigue Crack Propogation in Nitinol.,xe2x80x9d JBMR, 47,301-308,1999). This aspect, in combination with typical file designs that generally have inherently sharp notches, results in a fatigue life governed primarily by fatigue crack propagation. Under these conditions, NiTi actually has a lower fatigue threshold than other titanium alloys. For example, the fatigue threshold for pure titanium (beta) is about 10 MPa-m0 5 while that for NiTi is only about 2 MPa-m0.5. Even 316 stainless steel has a fatigue threshold of about 6 MPa-m0.5 (Ritchie and McKelvey, JBMR, 47, 1999). Thus, all metal files will have a finite life before eventually breaking. Table 1 below compares the number of revolutions to breakage, for a simulated canal root with a 90xc2x0 bend, for stainless steel (K file) and NiTi (K file) files (Nickel-Titanium Instruments, Serene, et. al., 1995).
Due to the high (super-elastic) flexibility of NiTi, it is difficult to machine or grind the flutes and other helical cutting features into a file or instrument. There are several patents that describe methods for grinding NiTi and other titanium alloys with more than 40 percent titanium. These patents are U.S. Pat. Nos. 5,464,362, 5,941,760, and 5,762,541, by Heath and Mooneyhan and U.S. Pat. No. 5,984,679 by Farzin-Nia and Otsen.
There have been other approaches to reducing the tendency for breakage such as relocating the helical recesses (U.S. Pat. No. 5,106,298 by Heath and Mooneyhan), brazed metal composites (U.S. Pat. No. 5,927,912 by Mihai and Erpenbeck), and abi-metallic file with an inner metallic core to vary modulus (U.S. Pat. No. 5,380,200 by Heath and Berendt). Lower friction coatings have been proposed as well as design methods to reduce torsional force on the file. All of the above design and coating related approaches to reduce the tendency for file breakage are now well known in the art and are included within the scope of the present invention.
It should be noticed that all prior art relative to endodontic files relates only to solid metallic files. There is no prior art that describes the use of multi-fiber composite. It is an object therefore, of the present invention to provide a method of making an endodontic dental instrument, using fiber composite methods, that has low stiffness (good flexibility) and improved resistance to gross breakage.
The present invention is directed to an improved endodontic instrument incorporating any design feature, that can be tubular, and can have lubricating surface coatings. Unlike the currently used metal files, the inventive instrument is comprised of a plurality of fibers having a flexible polymer matrix. The polymer matrix can incorporate all fibers or only fibers near the outside diameter of the instrument. The fibers can be present as a core or present throughout the instrument. When present as a core, fiber-free polymer can comprise a portion of the surface and surface features. The polymer can have abrasive particles embedded within, at the surface or both, in order to improve cutting efficiency and to reduce wear of the polymer or polymer-fiber surface region.
The basic concept is similar to twisting a rope (the fiber) and freezing it in place (via the polymer matrix). The rope can be twisted and pushed but it can""t xe2x80x9cbreak.xe2x80x9d The inventive twisted fiber endodontic instrument is unique and distinguishable from that described in the prior art.